Electron beam scanning imaging is a commercial method of generating electronic images. In electron beam scanning imaging systems, an electron beam scans a charge pattern on a target, for example, a photoconductor or photoemissive target. The particular point being scanned on a target is discharged, the degree of discharge indicating the intensity of the image.
In the Zworkin Iconoscope, a high energy electron beam scanned a photoemissive, insulating target, emitting a low energy secondary electron or electrons. The Zworkin Iconoscope was followed by the Image Iconoscope and the Superemitron.
In the Image Iconoscope and the Superemitron, the image was focused upon a semitransparent, conducting photocathode whereby to emit secondary electrons. These secondary electrons, that is, photoelectrons, were focused on an insulating target. The insulating target was scanned with high energy electrons.
The Image Iconoscope and the Superemitron were followed by the Image Orthocon where the image was focused on a photocathode which emitted secondary electrons to one side of a two sided target screen. The reverse side of the two sided target screen was scanned by low energy electrons. The Image Orthocon was replaced by the Image Isocon and thereafter by the Vidicon. In the Vidicon, the image was formed by positive charges on a photoconductor target. The thusly formed positive charges were discharged by the scanning electron beam and the image signal taken out at the target. In this way, a narrow electron beam was utilized to provide high resolution.
One problem with electron scanned imaging systems is the size, including volume and weight, of the tube. Another problem with electron scanned imaging systems is the complex electronic circuit needed to guide the electron beam.
Alternative methods of image detection and formation are the solid state, self-scanned array and the charge coupled device. Typically, self-scanned arrays are crystalline or polycrystalline mosaic sensors which operate by switching rather than by moving an electron beam.
The solid state self-scanning arrays contain an array of photosensitive elements. Each element is at an intersection of mutually perpendicular address buses. The entire array is exposed to incident light, and the photosensitive elements are scanned sequentially or in parallel by an addressing circuit rather than by a moving electron beam. Various circuit elements may be used to provide the necessary array of photosensitive elements and switching elements. For example, the combination of photoactive element and switching element may be a photoconductor and diode, a photodiode and diode, a phototransistor, on a photodiode and field effect transistor. In one mode of operation a photoconductor charges a capacitor and the addressing circuit discharges the capacitor, the current on discharge being a function of the intensity of light falling at the intersection.
Charge coupled devices depend on the collective transfer of mobile electric charge stored in semiconductor storage elements. The electric charge is in the form of electrons generated by the Einstein photoelectric effect. The collective transfer occurs in a serial or "bucket brigade" manner under the control of a clock pulse. The clock pulse forms and displaces a potential well of positive charge. The displacement of the potential well by the clock pulse causes photoelectrically agenerated electrons to move from individual cell to individual cell.
Charge coupled device imaging is carried out by holding the potential wells stationary during an integration period. During this integration period electrons are photoelectrically generated in each photosensor cell. Each photosensor cell has a charge coupled device shift register cell associated with it. At the end of the integration period all of the charge stored in the photosensor cells is transferred to the shift register cells associated therewith, and another integration period is started. Meanwhile, the charge in each charge coupled device shift register cell is read out in serial or bucket brigade manner under the control of the clock, thereby generating a video signal.
Self-scanned solid state arrays and charge coupled devices suffer from the problems inherent in crystalline semiconductor technology, i.e. the maximum size of the sensor is limited by the limtations on epitaxily grown semiconductors and the problems inherent in a multiplicity of photolithographic steps necessary to fabricate the solid state devices.